The invention relates to reflective ferroelectric liquid crystal-based light valves such as those used in video displays and in particular relates to such light valves for forming color images and having a substantially increased light throughput.
A need exists for various types of color video and graphics display devices with improved performance and lower cost. For example, a need exists for miniature color video and graphics display devices that are small enough to be integrated into a helmet or a pair of glasses so that they can be worn by the user. Such wearable color display devices would replace or supplement the conventional displays of computers and other devices. A need also exists for a replacement for the conventional cathode-ray tube used in many display devices including computer monitors, conventional and high-definition television receivers and large-screen displays. Both of these needs can be satisfied by display devices that incorporate a light valve that uses as its light control element three reflective spatial light modulators, each based on a ferroelectric liquid crystal (FLC) material.
A FLC-based spatial light modulator is composed of a layer of a FLC material, preferably a surface-stabilized FLC material, sandwiched between a transparent electrode and a reflective electrode that is segmented into an array of pixel electrodes to define the picture elements (pixels) of the spatial light modulator. The reflective electrode is located on the surface of a silicon substrate that also accommodates the drive circuits that derive the drive signals for the pixel electrodes from an input video signal.
The direction of an electric field applied between each pixel electrode and the transparent electrode determines whether or not the corresponding pixel of the spatial light modulator rotates the direction of polarization of light reflected by the pixel. The reflective spatial light modulator is constructed as a quarter-wave plate so that the polarized light reflected by the pixels of the spatial light modulator is either rotated by 90xc2x0 or not depending on the direction of the electric field applied to each pixel. A polarization analyzer is in the optical path of the light reflected by the spatial light modulator. The polarization analyzer is aligned to either: 1) transmit the polarized light which has rotated and absorb the polarized light which as not been rotated; or 2) transmit the polarized light which as not rotated and to absorb the polarized light which has been rotated. The resulting optical characteristics of each pixel of the spatial light modulator are binary: the light reflected by the pixel either is transmitted through the polarization analyzer (its 1 state) or is absorbed by the polarization analyzer (its 0 state), and therefore appears light or dark, depending on the direction of the electric field.
To produce the grey scale required for conventional display devices, the apparent brightness of each pixel is varied by temporally modulating the light transmitted by each pixel. The light is modulated by defining a basic time period that will be called the illumination period of the spatial light modulator. The pixel electrode is driven by a drive signal that switches the pixel from its 1 state to its 0 state. The duration of the 1 state relative to the duration of the illumination period determines the apparent brightness of the pixel.
Ferroelectric liquid crystal-based spatial light modulators suffer the disadvantage that, after each time the drive signal has been applied to a pixel electrode to cause the pixel to modulate the light passing through it, the DC balance of the pixel must be restored. This is typically done by defining a second basic time period called the balance period, equal in duration to the illumination period, and driving the pixel electrode with a complementary drive signal having 1 state and 0 state durations that are complementary to the 1 state and 0 state durations of the drive signal during the illumination period. The illumination period and the balance period collectively constitute a display period. To prevent the complementary drive signal from causing the display device to display a substantially uniform, grey image, the light source illuminating the light valve is modulated, either directly or with a shutter, so that the light valve is only illuminated during the illumination period, and is not illuminated during the balance period. However, modulating the light source as just described reduces the light throughput of the light valve to about half of that which could be achieved if DC balance restoration were unnecessary. This means that a light source of approximately twice the intensity, with a corresponding increase in cost, is necessary to achieve a given display brightness. Additionally or alternatively, projection optics with a greater aperture, also with a corresponding increase in cost, are necessary to achieve a given brightness.
FIG. 1A shows part of a conventional display device 5 incorporating a conventional reflective light valve 10 that includes the reflective spatial light modulator 12. Other principal components of the light valve are the polarizer 14, the beam splitter 16 and the analyzer 18. The light valve is illuminated with light from the light source 20, the light from which is concentrated on the polarizer using a reflector 22 and collector optics 24. The light output by the light valve passes to the imaging optics 26 that focus the light to form an image (not shown). The light valve 10, light source 20 and imaging optics may be incorporated into various types of display device, including miniature, wearable devices, cathode-ray tube replacements, and projection displays.
Light generated by the light source 20 enters the light valve 10 by passing through the polarizer 14. The polarizer polarizes the light output from the light source. Alternatively, a polarized light source (not shown) can be used and the need for the polarizer 14 would be eliminated. The beam splitter 16 then reflects a fraction of the polarized light output from the polarizer towards the spatial light modulator 12. The beam splitter can additionally or alternatively be a polarizing beam splitter configured to reflect light having a direction of polarization parallel to the direction of polarization of the polarizer 14 towards the spatial light modulator 12. The spatial light modulator 12 is divided into a two-dimensional array of picture elements (pixels) that define the spatial resolution of the light valve 10. Light reflected from the spatial light modulator can pass to the beam splitter 16 which transmits a fraction of the reflected light to the analyzer 18. If the beam splitter is a polarizing beam splitter, however, only light having a direction of polarization orthogonal to the direction of polarization imparted by the polarizer will be transmitted and the need for an independent analyzer would be eliminated.
The direction of an electric field in each pixel of the spatial light modulator 12 determines whether or not the direction of polarization of the light reflected by the pixel is rotated by 90xc2x0 relative to the direction of polarization of the incident light. The light reflected by each pixel of the spatial light modulator passes through the beam splitter 16 and the analyzer 18 and is output from the light valve 10 through the imaging optics 26 depending on whether or not its direction of polarization was rotated by the spatial light modulator.
More specifically, the polarizer 14 polarizes the light generated by the light source 20 that passes through the collector optics 24 either directly or after reflecting off reflector 22. The polarization is preferably linear polarization. The beam splitter 16 reflects the polarized light output from the polarizer towards spatial light modulator 12, and the polarized light reflected from the spatial light modulator transmits to the analyzer 18 through the beam splitter 16. The direction of maximum transmission of the analyzer is orthogonal to that of the polarizer in this example.
For purposes of this description, the terms parallel and orthogonal will be used to describe directions of polarization and directions of maximum transmissivity and maximum reflectivity. When these terms are used within this description, it is understood that they relate to the optical characteristics of the light and of the various components that comprise the light valve and not necessarily to their spatial relationships. For example, when polarized light reflects from a mirror at an angle of incidence of 45xc2x0, the polarized light is reflected at an angle of 90xc2x0 relative to the incident light. Even though the incident light and the reflected light are spatially orthogonal to one another, the direction of polarization of the reflected light will be optically unchanged from the direction of polarization of the incident light. Thus, the direction of polarization of the reflected light may be said to be parallel to the direction of polarization of the incident light. In addition, as used herein the term parallel shall include directions that are both parallel and anti-parallel, i.e., having a direction 180xc2x0 opposed, to the original direction.
The spatial light modulator 12 is composed of a transparent electrode 28 deposited on the surface of a transparent cover 30, a reflective electrode 32 located on the surface of the semiconductor substrate 34, and a ferroelectric liquid crystal layer 36 sandwiched between the transparent electrode 28 and the reflective electrode 32. The reflective electrode is divided into a two-dimensional array of pixel electrodes that define the pixels of the spatial light modulator and of the light valve. A substantially reduced number of pixel electrodes are shown to simplify the drawing. For example, in a light valve for use in a large-screen computer monitor, the reflective electrode could be divided into a two-dimensional array of 1600xc3x971200 pixel electrodes. An exemplary pixel electrode is shown at 38. Each pixel electrode reflects the portion of the incident polarized light that falls on it towards the beam splitter 16.
A drive circuit (not shown), which may be located in the semiconductor substrate 34, applies a drive signal to the pixel electrode 38 of each pixel of the spatial light modulator 12. The drive signal has two different voltage levels, and the transparent electrode 28 is maintained at a fixed potential mid-way between the voltage levels of the drive signal. The potential difference between the pixel electrode and the transparent electrode establishes an electric field across the part of the liquid crystal layer 36 between the pixel and transparent electrodes. The direction of the electric field determines whether the liquid crystal layer rotates the direction of polarization of the light reflected by the pixel electrode, or leaves the direction of polarization unchanged.
Since light passes through the reflective spatial light modulator twice, once before and once after reflection by the reflective pixel electrodes, the reflective spatial light modulator 12 is structured as a quarter-wave plate. The thickness of the layer of ferroelectric liquid crystal material in the liquid crystal layer 36 is chosen to provide an optical phase shift of 90xc2x0 between light polarized parallel to the director of the liquid crystal material and light polarized perpendicular to the director. The liquid crystal material is preferably a Smectic C* surface stabilized ferroelectric liquid crystal material having an angle of 22.5xc2x0 between its director and the normal to its smectic layers. Reversing the direction of the electric field applied to such a liquid crystal material switches the director of the material through an angle of about 45xc2x0. Consequently, if the director is aligned parallel to the direction of maximum transmission of the analyzer 18 with one polarity of the electric field, reversing the direction of the electric field will rotate the direction of polarization of light reflected by the pixel through 90xc2x0. This will align the direction of polarization of the light perpendicular to the direction of maximum transmission of the analyzer, and will change the pixel from its 1 state, in which the pixel appears bright, to its 0 state, in which the pixel appears dark.
In a miniature, wearable display, the imaging optics 26 are composed of an eyepiece that receives the light reflected by the reflective electrode 32 and forms a virtual image at a predetermined distance in front of the user (not shown). In a cathode-ray tube replacement or in a projection display, the imaging optics are composed of projection optics that focus an image of the reflective electrode on a transmissive or reflective screen (not shown). Optical arrangements suitable for use as an eyepiece or projection optics are well known in the art and will not be described here.
Since the direction of maximum transmission of the analyzer 18 is orthogonal to the direction of polarization defined by the polarizer 14, light whose direction of polarization has been rotated through 90xc2x0 by a pixel of the spatial light modulator 12 will pass through the analyzer and be output from the light valve 10 whereas light whose direction of polarization has not been rotated will not pass through the analyzer. The analyzer only transmits to the imaging optics 26 light whose direction of polarization has been rotated by pixels of the spatial light modulator. The pixels of the spatial light modulator will appear bright or dark depending on the direction of the electric field applied to each pixel. When a pixel appears bright, it will be said to be in its 1 state, and when the pixel appears dark, it will be said to be in its 0 state.
The direction of maximum transmission of the analyzer 18 can alternatively be arranged parallel to that of the polarizer 14, and a non-polarizing beam splitter can be used as the beam splitter 16. In this case, the spatial light modulator 12 operates in the opposite sense to that just described.
To produce the grey scale required by a display device notwithstanding the binary optical characteristics of the pixels of the light valve 10, the apparent brightness of each pixel is varied by temporally modulating the light reflected by the pixel, as described above. The drive circuit (not shown) for each pixel of the spatial light modulator determines the duration of the 1 state of the pixel in response to a portion of the input video signal 40 corresponding to the location of the pixel in the spatial light modulator.
FIGS. 1B-1F illustrate the operation of the exemplary pixel 38 of the conventional light valve 10 shown in FIG. 1A during three consecutive display periods. The remaining pixels operate similarly. In one embodiment of a conventional light valve, each display period corresponded to one frame of the input video signal 40. In another embodiment, each display period corresponded to a fraction of one frame of the input video signal. Each display period is composed of an illumination period (ILLUM) and a balance period (BALANCE) having equal durations, as shown in FIG. 1B.
FIG. 1C shows the drive signal applied to the exemplary pixel electrode 38. The transparent electrode 28 is held at a voltage level of V/2, so that changing the voltage level on the pixel electrode from 0 to V reverses the direction of the electric field applied to the ferroelectric liquid crystal layer 36. The level of the drive signal is V for a first temporal portion 1TP of each illumination period. The level of the drive signal is 0 for the second temporal portion 2TP constituting the remainder of the illumination period, and also for the first temporal portion 1TP of the subsequent balance period. The first temporal portion of the balance period has a duration equal to the first temporal portion of the illumination period. However, the level of the drive signal is 0 during the first temporal portion of the balance period, whereas the level of the drive signal is V during the first temporal portion of the illumination period. Finally, the level of the drive signal changes to V for the second temporal portion 2TP constituting the remainder of the balance period. Consequently, during the balance period, the level of the drive signal is 0 and V for times equal to the times that it was at V and 0, respectively, during the illumination period. As a result, the electric field applied to the liquid crystal material of the pixel averages to zero over the display period.
In the example shown, the duration of the first temporal portion 1TP of the drive signal is different in each of the three illumination periods. The duration of the first temporal portion, and, hence, of the second temporal portion, of each illumination period depends on the voltage level of the corresponding sample of the input video signal 40.
FIG. 1D shows the effect of the spatial light modulator 12 on the direction of polarization of the light impinging on the analyzer 18. The direction of polarization is indicated by the absolute value of the angle xcex1 between direction of polarization of the light impinging on the analyzer and the direction of maximum transmissivity of the analyzer. The analyzer transmits light having an angle xcex1 close to zero and absorbs light having an angle xcex1 close to 90xc2x0. In each display period, the angle xcex1 has values corresponding to the pixel being bright and dark for equal times due to the need to restore the DC balance of the pixel.
FIG. 1E shows the condition of a fast-acting light source 20. The light source is ON throughout the illumination period of each display period, and is OFF during the following balance period. Alternatively, the light source could remain on and a shutter (not shown) could be used to control whether the light generated by the light source illuminates the spatial light modulator 12. For example, an open shutter would correspond to the light source being ON and a closed shutter would correspond to the light source being OFF.
FIG. 1F shows the light output from the exemplary pixel of the light valve 10 controlled by the pixel electrode 38. Light is output from the pixel only during the first temporal portion of the illumination period of each display period. No light is output during the second temporal portion of the illumination period. Moreover, no light is output during the balance period of the display period because the light source 20 is OFF during the balance period.
The light valve 10 shown in FIG. 1A can also be adapted to provide a colored light output to the imaging optics 26. One way that this can be done is by replacing the xe2x80x9cwhitexe2x80x9d light source 20 with three colored light sources such as a red, blue and green LEDs (not shown), each illuminating the spatial light modulator 12 sequentially. This would require a balance period after each sequential illumination period. Another way that a colored light output can be provided is by replacing the single reflective spatial light modulator 12 shown in FIG. 1A with three reflective spatial light modulators and a color separator for separating the light into three component colors.
An example of one such color configuration is depicted in FIG. 2. Regarding this figure and those that follow, it is noted that identical reference numerals are used to designate identical or similar elements throughout the several views, and that elements are not necessarily shown to scale. In FIG. 2, the color separator is a series of three dichroic plates 42,43,44, each having an associated reflective spatial light modulator 12. Each of the dichroic plates is configured to reflect light in a band of wavelengths (colorband) particular to that dichroic plate and to pass the remaining wavelengths of light. Thus, if the light source 20 is a xe2x80x9cwhitexe2x80x9d light, emitting visible light across the entire visible color spectrum, a particular portion of the color spectrum may be reflected by each dichroic plate its associated reflective spatial light modulator simultaneously. This eliminates the need for sequential illumination and improves the perceived brightness of the color pixels passing through the analyzer.
For example, the dichroic plate 42 nearest the beam splitter 16 might reflect red-colored light toward its associated spatial light modulator 12 while the center dichroic plate 43 reflects green-colored light toward its associated spatial light modulator and the dichroic plate remote from the beam splitter 44 reflects blue-colored light towards its spatial light modulator. When the light source 20 is ON, as shown if FIG. 2, the colored light reflected by the dichroic plates passes to each of the three reflective spatial light modulators 12. Each of the three reflective spatial light modulators is capable of reflecting pixels of the colored light back at its associated dichroic plate in a manner consistent with the above description of the operation of the spatial light modulator shown in FIG. 1A.
The majority of the colored light reflected by each of the spatial light modulators 12 will be reflected by its associated dichroic plate toward the analyzer 18 since the light reflected by each spatial light modulator will retain the characteristic wavelengths of light originally reflected by its respective dichroic plate. When the combined colored light from each of the three reflective spatial light modulators 12 passes through the analyzer, a full color image can be formed by the imaging optics 26.
FIG. 3 depicts the use of a color separation cube 46, sometimes known as an x-cube or crossed-dichroic cube, as a color separator in place of the three dichroic plates. As with the three dichroic plates, the color separation cube separates three distinct color bands from the xe2x80x9cwhitexe2x80x9d light created by light source 20 and directs each of the color bands to a particular spatial light modulator 12. The color separation cube 46 also recombines the light reflected from each of the spatial light modulators 12 and directs the combined light toward the analyzer 18. The use of a color separation cube allows for a more compact design utilizing three spatial light modulators than can be achieved using three separate dichroic plates.
FIG. 4 depicts the use of a third type of color separator, a three-prism color separator 48 (sometimes known as a Philips cube or Philips prism), in a light valve utilizing three spatial light modulators to generate a color image. The design and use of a three-prism color separator is described in detail in U.S. Pat. No. 5,644,432, the contents of which are incorporated herein by reference. Like the previously described color separators, the three-prism color separator separates three distinct color bands from the xe2x80x9cwhitexe2x80x9d light created by light source 20 and directs each of the color bands to a particular spatial light modulator 12. The three-prism color separator 48 also recombines the light reflected from each of the spatial light modulators 12 and directs the combined light toward the analyzer 18. The three-prism color separator has the advantage over the three dichroic plates and the color separation cube in that it typically does a better job of recombining the reflected light from each of the spatial light modulators into a single color image.
While the use of dichroic plates, a color separation cube, or three-prism color separator allows color images to be formed by three ferroelectric liquid crystal-based spatial light modulators and a xe2x80x9cwhitexe2x80x9d light source, each of the spatial light modulators still suffers the disadvantage that the DC balance of the pixel must be restored. As with the single spatial light modulator discussed above, DC balance is typically achieved by defining a balance period and driving the pixel electrode with a complementary drive signal having 1 state and 0 state durations that are complementary to the 1 state and 0 state durations of the drive signal during the illumination period. The xe2x80x9cwhitexe2x80x9d light source is typically modulated during the balance period to prevent the complementary drive signal from causing the display device to display a substantially uniform, grey image. Modulating the light source as just described, however, reduces the light throughput of the light valve to about half of that which could be achieved if DC balance restoration were unnecessary. This means that a light source of approximately twice the intensity, with a corresponding increase in cost, is necessary to achieve a given display brightness. Additionally or alternatively, projection optics with a greater aperture, also with a corresponding increase in cost, are necessary to achieve a given brightness.
Consequently, what is needed is a color light valve utilizing three reflective ferroelectric liquid crystal spatial light modulators that can remain illuminated during the balance period so that the light throughput of the light valve can be approximately twice that of a conventional, color light valve utilizing three spatial light modulators.
The invention provides a high throughput color light valve that comprises a light input, a light output, a beam splitter, a color separator, a switchable half-wave plate, and reflective ferroelectric liquid crystal based spatial light modulators. Light having a direction of polarization parallel to a first direction is received through the light input. The light received at the light input is output from the light output after reflection by at least one of the spatial light modulators. The reflective spatial light modulators are each structured as a quarter-wave plate and each has a principal axis that independently switches through an angle of rotation substantially equal to xcfx86. The color separator is configured to separate the light received at the light input into colorbands, and also to distribute the colorbands to the spatial light modulators. The beam splitter is located and aligned relative to the light input, the light output, the color separator and the spatial light modulators either to reflect or transmit the light received at the light input towards the color separator, and transmit or reflect, respectively, towards the light output the light reflected by the spatial light modulators. The switchable half-wave plate is located between the beam splitter and the color separator, is structured as a half-wave plate, and has a principal axis that switches through an angle of rotation xcex8 substantially equal to xcfx86/2.
The spatial light modulators may include a first, second, and third spatial light modulator, and the color separator may separate a first, second, and third color waveband from the light received at the light input. The beam splitter may include a polarizing beam splitter with orthogonal directions of maximum transmissivity and maximum reflectivity, one of the directions being parallel to the first direction.
The switchable half-wave plate may include a pair of opposed transparent electrodes and a layer of liquid crystal material sandwiched between the electrodes. The liquid crystal material may be a ferroelectric liquid crystal material or a nematic liquid crystal material. Alternatively, the direction of the principal axis of the switchable half-wave plate may be mechanically switched.
The color separator may include three dichroic plates, a color separation cube, or a three-prism color separator.
The invention also provides a method of increasing the light throughput of a multi-color component reflective light valve that requires DC balancing. In the method, a reflective light valve is provided that includes a first, second, and third reflective spatial light modulator and a polarizing beam splitter. The polarizing beam splitter has orthogonal directions of maximum transmissivity and maximum reflectivity, one of which defines the direction of polarization of light incident on the reflective spatial light modulators, the other of which defines the direction of polarization of light output from the light valve. Each of the spatial light modulators has a principal axis independently switchable between a first direction and a second direction. The second direction is substantially at an angle xcfx86 to the first direction. Also provided is a switchable half-wave plate that has a principal axis switchable between a third direction and a fourth direction. The fourth direction is at an angle xcex8 to the third direction.
The switchable half-wave plate is inserted into the light valve between the polarizing beam splitter and the color separator with the third direction aligned parallel to the first direction. The spatial light modulators are operated in a first time period and a second time period equal to the first time period with each principal axis independently in the first direction for a portion of the first time period, in the second direction for the remainder of the first time period and a portion of the second time period, and in the first direction for the remainder of the second time period. The portion of the second time period is equal in duration to the portion of the first time period, and the portion of the second time period and the remainder of the second time period are in any temporal order. The switchable half-wave plate is operated with its principal axis in the third direction through the first period and in the fourth direction through the second period, and with the angle xcex8 substantially equal to xcfx86/2.
A switchable half-wave plate may be provided in which the principal axis switches between the third direction and the fourth direction in a switching time, and the method may additionally comprise illuminating the light valve with light, and reducing the intensity of the light during the switching time of the switchable half-wave plate.
Switching the switchable half-wave plate inverts the sense of the light valve relative to the direction of the electric field applied to the liquid crystal material of the spatial light modulator. When the first time period and the second time period correspond to the illumination period and the balance period of a display period, inverting the sense of the light valve during the balance period enables a display device incorporating the light valve to generate a positive image in both the illumination period and the balance period of the display period. Accordingly, the light valve can be illuminated during both the illumination period and the balance period. This almost doubles the light throughput of the light valve according to the invention compared with a conventional light valve.